Many bourbons are suggested here as good for mixing with soda pop or in cocktails. But more precisely, what does it mean to be a mixing bourbon, what types of bourbons are these (apart from being inexpensive)?

In my view, a good mixing bourbon should be defined more by what it is not than what it is: it should not be corny-tasting (the fresh corn oil taste many very young bourbons have), or too old - excess wood does not suit these drinks I feel - or too congeneric. The last means, we don't want it strongly chemical-tasting, i.e., a little is okay, perhaps even desirable. If it is none of these and the label states it is straight bourbon or rye, it will likely suit any mixed drink or cocktail. It may be good, too, for neat drinking, but that is a narrower category of these straight whiskeys.

If 3-4 years old is too young and 12+ years is too old, then logically you should be looking in the 6-10 year old range. I'd start with something like the Jim Beam Black or Wild Turkey 8 year olds and go from there.

High enough proof and big enough flavor to stand up to the other ingredients and dilution in the cocktail. If whiskey is the dominant ingredient, it should be slightly forward in the mix. If the drink tends sweet, pick a drier whiskey to add balance. Certain fruits and syrups lend themselves to particular whiskies. One of the more obvious ones to me is raspberry and rye. Those twin well. I think extra aged whiskies can be good in cocktails but the wood can be tricky particularly when mixing with amari and bitters. Can go sour on the finish.

"There exist mighty dogs, the dangerous kind who take hold of your heart and do not let go."